A laser measurement apparatus (for example, a laser radar), which is an example of a distance measurement apparatus, is not limited to measuring the distance to a target from a person measuring, and is used, for example, to detect an obstacle from a vehicle, detecting the presence of a person between a carriage and an sliding door on a railway station platform, and in a monitoring system.
For a laser measurement apparatus, there is a one-dimensional scanning scheme and a two-dimensional scanning scheme. In the one-dimensional scanning scheme, the direction of the projection of the laser beam is varied on a single plane while projecting the laser beam at a given time interval and thus, scanning may be performed on a single plane at a given angular interval. The two-dimensional scanning scheme may be implemented by, for example, performing the one-dimensional scanning sequentially on plural planes. The distance to an object may be calculated using either one of the one-dimensional scanning scheme and the two-dimensional scanning scheme by, for example, measuring the time of flight (TOF), which is the round-trip time taken for a projected laser beam to reach a target, be reflected, and return, and then multiplying the TOF by the speed of light and dividing the result thereof by two.
When a surrounding environment is being monitored by using a laser measurement apparatus from a vehicle, for example, when the vehicle is driving forward on a road, it is desirable that an angular interval which controls the projection angle of the laser beam is comparatively narrow and a scanning angle resolution is high in order to easily detect an obstacle ahead of the vehicle. However, for example when the vehicle is travelling backwards to park the vehicle, because the vehicle is reversing a relatively slow speed, it is desirable that the accuracy of a measurement device is set to be high so that the vehicle does not contact, for example, an obstacle behind the vehicle. In other words, the accuracy of measuring distance desired when, for example, the vehicle is travelling forward, because the distance to an object being measured is relatively long and there is a time to spare, the accuracy desired for distance measurement may be lower than the accuracy for distance measurement when, for example, the vehicle is being parked and the distance to an object being measured is relatively small. However, in order to make the accuracy of a measured distance higher, for example, simply widening the angular interval which controls the projection angle of the laser beam will not a produce a high scanning angle resolution.
Circuit noise is known as a cause that reduces the accuracy of a measured distance. Accordingly, in an effort to reduce the noise, a method in which a noise countermeasure is used in a circuit that calculates a measured distance may be considered. However, a complicated circuit is needed and thus, the cost for the distance measurement apparatus increases. Alternatively, a method in which samplings of the TOFs are repeated and averaged to improve the accuracy of a measured distance may be considered. However, in this case, since an update of distance data is delayed by the number of samples subjected to averaging, it takes a relatively longer time to calculate the measured distance. In order to reduce the time required to update the distance data, it is desirable to significantly narrow the angular interval which controls the projection angle of the laser beam as well as to shorten the time interval over which the laser beam is projected. However, a laser, which has a very short time interval (that is, the light emitting interval) over which a laser beam is projected, and an associated driving circuit is expensive. Accordingly, an expensive and complicate circuit may be needed to significantly narrow the angular interval which controls the projection angle of the laser beam and thus, the cost for the distance measurement apparatus increases.
As described above, there is a tradeoff between the accuracy of a measured distance and the scanning angle resolution in a conventional distance measurement apparatus and thus, it is difficult to improve the accuracy of a measured distance without sacrificing the scanning angle resolution and also difficult to improve the scanning angle resolution without sacrificing the accuracy of a measured distance. As the prior art documents, see, for example, Japanese Patent Application Laid-Open No. 2007-278940, Japanese Patent Application Laid-Open No. H3-176685, Japanese Patent Application Laid-Open No. 2011-2368, and Japanese Patent Application Laid-Open No. H11-202051.
It is difficult to improve the accuracy of a measured distance and the scanning angle resolution with a relatively simple circuit in a conventional distance measurement apparatus.
Accordingly, the present disclosure intends to provide a distance measurement apparatus, a distance measurement method and a program thereof that are capable of improving the accuracy of a measured distance and the scanning angle resolution with a relatively simple circuit.